It is often necessary in industrial and other applications to supply an air flow at relatively closely controlled humidity and temperature conditions. One such application is in the supply of air to paint spray booths wherein water-based paints are to be applied. Such processes typically require a 75.degree. F. dry bulb air temperature and 50% relative humidity, i.e., 55.degree. F. wet bulb temperature.
The use of such water-based paints in such applications as automotive paint spraying has become more widespread due to the lessening of the pollution problems associated with hydrocarbon solvent-based painting operations.
Such controlling of temperature and humidity has heretofore required relatively extravagant expenditures of energy. For example, if the air temperature in the summertime is at a higher temperature than required, and if the moisture content is also above the required level (as would typically be the case for summertime weather conditions), both the temperature and moisture content of the air must be reduced. If such moisture is removed mechanically, i.e., by passing such air through a heat exchanger where it is chilled to the appropriate dew point, i.e., 55.degree. F. as per the example given, the air flow must then be reheated to the required 75.degree. F. dry bulb temperature.
Given the enormous volumes of air flow which must be conditioned for typical automotive paint spraying installations, i.e., of the order of 100,000 CFM for each 10 foot length of spray booth, such energy expenditures become truly significant.
For humidifying, moisture may readily be added to flowing air mass by the use of high efficiency spray nozzles directed over a heat exchanger in which the air can be saturated, i.e., to 100% relative humidity. It is difficult however to control humidification by this process at lower levels, i.e., to 50% relative humidity.
Such addition of moisture to the air generally produces an evaporative cooling of the air mass such that dry bulb temperature may be below the required temperature after humidifying, thus requiring reheating.
There has heretofore been proposed and practiced dehumidification processes which do not cool the total air mass to the required dew point, in the interest of achieving improved efficiency. In such processes, a portion of the air flow is bypassed around the cooling coils such that only a portion of the air flow is cooled to a lower dew point temperature. This air mass thus is reduced to a lesser humidity level than required such that upon remixing with the bypass air flow, the combined mixture of the air flow is at the appropriate humidity and temperature condition.
Air conditioning systems of this type typically must accommodate great temperature and humidity variations in the ambient air, and the degree of modulation of the bypass air flow must be relatively great in order to achieve the final controlled air condition. The necessity of a relatively large modulation of the bypass air flow volume necessitates a complex damper system associated with the cooling coils and the bypass passage and greatly increases the bulk of the necessary ducting.
Also, the air flow characteristics of the system are difficult to properly balance particularly for high volume systems due to the flow resistance of the cooling coils.
For similar reasons, it is difficult to properly control the damper system modulation such as to closely and stably control the conditioned air flow if large variations in flow across the heat exchangers is required.
In many of these systems, it is difficult to achieve an accurately controlled humidification-dehumidification, cooling-heating process such that subsequent reheating is not required.
Such bypass flow arrangements must insure complete mixing of the bypass air with the conditioned air such as to avoid stratification in which different points within the air mass are at different humidity and temperature levels.
The present inventor's prior U.S. Pat. Nos. 4,173,125 ("Energy Recovery System") and 4,173,924 ("Paint Spray Booth With Air Supply System") disclose arrangements whereby relatively low grade heat energy may be recovered and utilized in air conditioning processes, in the interest of improving the overall efficiency of industrial processes. It is of course advantageous if any such air conditioning apparatus could utilize low grade energy heat source, or the energy value represented by exhausted cooled air, to enhance the efficiency of the conditioning process.
Accordingly, it is an object of the present invention to provide a method and apparatus of conditioning air to predetermined temperature and humidification levels which is highly efficient in the usage of energy in achieving such controlled levels.
It is yet another object of the present invention to provide such method and apparatus in which predetermined dry bulb temperature levels are achieved with a minimal amount of reheating of the air flow after mechanical dehumidification.
It is still another object of the present invention to provide a sprayed surface cooling-heating coil arrangement for carrying out the humidification-dehumidification and heating-cooling of the air flow in which the final humidity and temperature levels can be closely controlled.
It is yet another object of the present invention to provide a sprayed coil arrangement for humidification-dehumidification and cooling-heating of the air mass combined with a bypass flow damper for remixing unaltered air to achieve a given humidity and temperature level, in which the portion of damper controlled bypass flow necessary to achieve a given final condition is minimized.
It is still another object of the present invention to provide such sprayed coil humidification-dehumidification and heating-cooling of an air mass flowing therethrough with minimal bypass flow modulation necessary in order to accommodate varying ambient air temperature and humidity levels.
It is still another object of the present invention to provide a bypass ducting arrangement in which the air passing through the heat exchanger and the bypass passage are intimately mixed to provide a homogeneous conditioned air mass at appropriate temperature and humidity levels.
It is still another object of the present invention to provide such method and apparatus for achieving controlled humidity and temperature levels in which low grade energy recovered from waste heat sources and/or the energy value in cooled air to be rejected to the atmosphere are efficiently utilized in the conditioning process.
It is still a further object of the present invention to provide an air supply system for a paint spray booth in which ambient air either under summertime or wintertime conditions can be conditioned to appropriate temperature and humidity levels in a highly efficient manner, with a minimum of plant equipment.